Microbial communities in boreal coniferous forest humus exposed to heavy metals and changes in soil pH—a summary of the use of phospholipid fatty acids, Biolog® and 3H-thymidine incorporation methods in field studies

Abstract

This paper summarises our recent field studies on the microbial communities of boreal coniferous forest humus exposed to environmental stress, heavy metals and changes in humus pH. The microbial community was measured using the phospholipid fatty acid (PLFA) and Biolog® analyses, and the actual factor in the environment exerting the selective pressure on the bacterial community was estimated with the help of tolerance determinations using the 3H-thymidine incorporation technique. The field experiments showed that the structure of the microbial community inhabiting the boreal coniferous forest humus was influenced by changes in humus pH and heavy metal concentrations at levels where no, or only small, effects on the microbial biomass or carbon mineralization rate were seen. The alterations in the humus PLFA patterns were related to the abundance of the major groups of microorganisms, bacteria and fungi. Changes in the relative proportions of Gram-negative and Gram-positive bacteria, including actinomycetes, were also shown. With the help of the 3H-thymidine incorporation technique, it was demonstrated that forest humus bacterial communities exposed to heavy metals or alterations in humus pH were able to adapt to the environmental disturbance in question. When combining the results from the PLFA and 3H-thymidine analyses, it was revealed that the increased tolerance of the humus bacterial community to heavy metals or to altered pH resulted at least partly from a change in microbial species composition. Coniferous forest humus seemed to contain a bacterial group, consisting mainly of Gram-positive bacteria, which were adapted more easily to the acidifying environment and a group of bacteria, mainly Gram-negative ones, which were more easily adapted to the humus with a higher pH. The Biolog® technique, which determines the community level physiological profile (CLPP) of the bacterial community was less sensitive and less suitable than the PLFA analysis to detect the characteristics of the forest humus microbial community. The 3H-thymidine incorporation technique was the most sensitive of the techniques used in this study to detect the influence of environmental disturbances on the microbial community. In addition, a gradient of coniferous forest stands having naturally different humus pH because of the different site properties was studied to compare these natural microbial communities with the communities subjected to anthropogenic change in humus pH. In order to reveal the similarity of the humus samples with respect to their community structure, the PLFA patterns from all the field studies were subjected to multivariate cluster analysis. The structure of the forest humus microbial community was shown to be strongly influenced by the indigenous fertility of the coniferous forest site type, which was in turn related to humus nutritional status, pH, moisture, tree species and ground vegetation. Thus, a prerequisite for successful determination of the impacts of environmental stress on forest humus microbial community is the homogeneity of the forest site types between the experimental plots.